Medium carbon steel sheet for cold working and method for manufacturing the same

Abstract

A medium carbon steel sheet for cold working that has a hardness of 500 HV to 900 HV when subjected to high-frequency quenching and a quick cooling to a room temperature is carried out. The medium carbon steel sheet includes, by mass %, C: 0.30 to 0.60%, Si: 0.06 to 0.30%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.0075% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.01%, and Cr: 0.001 to 0.10%, the balance composed of Fe and inevitable impurities. An average diameter d of a carbide is 0.6 μm or less, a spheroidizing ratio p of the carbide is equal to or more than 70% and less than 90%, and the average diameter d (μm) of the carbide and the spheroidizing ratio p % of the carbide satisfy the expression d≦0.04×p−2.6.

The invention claimed is: 
     
       1. A medium carbon steel sheet for cold working that has a hardness of 500 HV to 900 HV in a case of being subjected to high-frequency quenching in which a temperature is raised at an average heating rate of 100° C./second, the temperature is held at 1,000° C. for 10 seconds, and a quick cooling to a room temperature is carried out at an average cooling rate of 200° C./second, the medium carbon steel sheet comprises,
 C: 0.30 to 0.60 mass %, 
 Si: 0.06 to 0.30 mass %, 
 Mn: 0.3 to 2.0 mass %, 
 P: 0.030 mass % or less, 
 S: 0.0075 mass % or less, 
 Al: 0.005 to 0.10 mass %, 
 N: 0.001 to 0.01 mass %, and 
 Cr: 0.001 to 0.10 mass %, 
 and a balance composed of Fe and inevitable impurities, wherein 
 an average diameter d of a carbide is 0.37 μm or less, a spheroidizing ratio p of the carbide is equal to or more than 70% and less than 90%, and the average diameter d in μm of the carbide and the spheroidizing ratio p in % of the carbide satisfy d≦0.04×p−2.6. 
 
     
     
       2. The medium carbon steel sheet for cold working according to  claim 1 ,
 further comprises one or more of, 
 Ni: 0.01 to 0.5 mass %, 
 Cu: 0.05 to 0.5 mass %, 
 Mo: 0.01 to 0.5 mass %, 
 Nb: 0.01 to 0.5 mass %, 
 Ti: 0.001 to 0.05 mass %, 
 V: 0.01 to 0.5 mass %, 
 Ta: 0.01 to 0.5 mass %, 
 B: 0.001 to 0.01 mass %, 
 W: 0.01 to 0.5 mass %, 
 Sn: 0.003 to 0.03 mass %, 
 Sb: 0.003 to 0.03 mass %, and 
 As: 0.003 to 0.03 mass %. 
 
     
     
       3. The medium carbon steel sheet for cold working according to  claim 2 ,
 wherein a Cr content [Cr mass %] and a Mo content [Mo mass %] satisfy [Cr mass %]+[Mo mass %]/10<0.10. 
 
     
     
       4. The medium carbon steel sheet for cold working according to  claim 1  or  2 ,
 wherein the hardness prior to cold working is greater than or equal to 120 HV and less than 170 HV.